Solar Thermal Energy Absorber Tube

ABSTRACT

An absorber tube is provided that includes an inner tube and an outer tube that at least partially surrounds the inner tube and is concentric with the inner tube. An expansion element is present and is configured for expanding in response to differences in thermal expansion between the inner tube and the outer tube. A getter is located radially at a position between the inner tube and the outer tube.

FIELD OF THE INVENTION

The present invention relates generally to absorber tubes used in the collection and use of solar thermal energy. More particularly, the present application involves an absorber tube with an improved design to handle thermal expansion thereof and a getter material to aid in the generation of a vacuum within the absorber tube.

BACKGROUND

A parabolic trough is a solar thermal energy collector that functions to convert thermal energy into mechanical energy. The parabolic trough includes a series of parabolic mirrors that are arranged into parallel rows. The mirrors may be made from polished aluminum and function to focus solar energy directed thereon to an absorber tube that runs the lengths of the rows of mirrors. The parabolic mirrors may be arranged so as to be capable of moving in order to be oriented towards the sun for maximum solar thermal energy collection.

A substance, such as oil or water, flows through the absorber tube and is heated by the parabolic mirrors. The heated substance is transported to a heat engine that in turn converts thermal energy stored in the heated substance into mechanical energy. The substance is located and transported within an inner tube of the absorber tube that is surrounded by a concentric outer tube. The outer tube is made of glass so that solar thermal energy directed by the parabolic mirrors is more easily directed therethrough and onto the inner tube that includes the substance. The space between the inner tube and the outer tube can be evacuated so that a vacuum is formed. The vacuum functions to reduce heat transfer from the heated substance and inner tube back to the outer tube. Preventing temperature rise of the outer tube increases the efficiency of the parabolic trough and also enhances safety as the outer tube may be cool to the touch even though the inner tube is extremely hot.

Temperature differences between the inner tube and outer tube present certain challenges in the design of absorber tubes. For example, the inner tube and outer tube may expand at different rates or amounts due to differences in the thermal properties of the material making up the inner and outer tubes. Thermal expansion compensation devices may be located at various points along the concentric tubes in order to compensate for thermal expansion between the tubes. Thermal expansion compensation devices may be arranged so that the inner and outer tubes are in sliding engagement with one another and may include bellows that function to control the movement between the expanding components. These components may include moving parts that can ultimately fail and cause a loss of vacuum within the absorber tube that can in turn lead to a decrease in the efficiency of the parabolic trough. As such, there remains room for variation and improvement in the art.

BRIEF DESCRIPTION OF THE DRAWINGS

A full and enabling disclosure of the present invention, including the best mode thereof, directed to one of ordinary skill in the art, is set forth more particularly in the remainder of the specification, which makes reference to the appended Figs. in which:

FIG. 1 is a perspective view of a parabolic trough in accordance with one exemplary embodiment.

FIG. 2 is a cross-sectional view of an absorber tube in accordance with one exemplary embodiment.

FIG. 3 is a cross-sectional view of the absorber tube of FIG. 2 in a thermally expanded condition.

FIG. 4 is a cross-sectional view of the absorber tube of FIG. 2 with a getter evaporated and forming a coating on the inner wall of the outer tube.

Repeat use of reference characters in the present specification and drawings is intended to represent the same or analogous features or elements of the invention.

DETAILED DESCRIPTION OF REPRESENTATIVE EMBODIMENTS

Reference will now be made in detail to embodiments of the invention, one or more examples of which are illustrated in the drawings. Each example is provided by way of explanation of the invention, and not meant as a limitation of the invention. For example, features illustrated or described as part of one embodiment can be used with another embodiment to yield still a third embodiment. It is intended that the present invention include these and other modifications and variations.

It is to be understood that the ranges mentioned herein include all ranges located within the prescribed range. As such, all ranges mentioned herein include all sub-ranges included in the mentioned ranges. For instance, a range from 100-200 also includes ranges from 110-150, 170-190, and 153-162. Further, all limits mentioned herein include all other limits included in the mentioned limits. For instance, a limit of up to 7 also includes a limit of up to 5, up to 3, and up to 4.5.

The present invention provides for an absorber tube 14 for use with a parabolic trough 10. The absorber tube 14 features an expansion element 34 that is used to minimizes stress imparted to the absorber tube 14 through thermal expansion. The absorber tube 14 may also include a getter 40 located between the outer tube 16 and inner tube 22 for use in maintaining a vacuum formed in the space formed between tubes 16 and 22. Additionally, the getter 40 may be provided in order to act as an indicating device to alert personnel of the loss of vacuum within the absorber tube 14 so that repair or replacement can then be conducted.

One exemplary embodiment of an absorber tube 14 as incorporated into a parabolic trough 10 is illustrated in FIG. 1. The parabolic trough 10 includes a parabolic mirror 12 that functions to focus sunlight imparted thereon onto an absorber tube 14. The parabolic trough 10 and absorber tube 14 may extend any length in the longitudinal direction 30. Additional parabolic troughs 10 and absorber tubes 14 can be located next to one another in rows to provide for additional solar energy collection. Further, the parabolic mirrors 12 and/or absorber tubes 14 can be made mobile so that their orientation with respect to the sky can be modified. In this manner, the parabolic trough 10 can be oriented with respect to the sun so as to achieve maximum solar collection. Heat collected by the absorber tube 14 is transported to a downstream location for utilization or storage. In accordance with certain embodiments the collected heat energy can be used in a single effect or in a double effect absorption chiller. The absorber tube 14 can be used in a variety of applications in accordance with various exemplary embodiments and it is to be understood that the absorber tube 14 is not limited to a specific application or limited in use with a parabolic trough 10 and/or parabolic mirror 12.

FIG. 2 shows an absorber tube 14 in accordance with one exemplary embodiment. The absorber tube 14 includes an outer tube 16 that has an outer wall 18 and an inner wall 20. The outer tube 16 may be made of borosilicate glass in accordance with one embodiment. The material making up the outer tube 16 may be selected so that it is capable of allowing light energy to pass therethrough with little or no resistance or reflection. An inner tube 22 is provided and includes an outer wall 24 and an inner wall 26. The outer tube 16 and inner tube 22 are concentric with one another about axis 28. A substance such as oil or water is present within the inner tube 22 and absorbs heat transferred from the parabolic mirror 12 though the outer tube 16 and into the inner tube 22. The substance is then transferred through the inner tube 22 to a desired location in which the heat contained therein can be utilized or stored. The outer wall 24 of the inner tube 22 may be coated with a metal or paint that affords enhanced solar radiation absorption and minimal reflection so that solar energy absorption by the inner tube 22 is encouraged. For example, in one arrangement the outer wall 24 may be coated with flat black paint. The inner tube 22 may be made from a variety of materials. In accordance with one exemplary embodiment inner tube 22 is made of stainless steel.

An expansion element 34 is located beyond an end 48 of the outer tube 16 in the longitudinal direction 30. In this regard, the expansion element 34 is located completely beyond the end 48 in the longitudinal direction 30 so that no portion of the expansion element 34 is located between the inner tube 22 and the outer tube 16. As shown, the expansion element 34 is located outward of the inner tube 22 in the radial direction 32 and surrounds a portion of the inner tube 22, but the expansion element 34 does not surround and is not surrounded by any portion of the outer tube 16 in the radial direction 32. The expansion element 34 includes a series of radially extending portions 36 that are connected by a series of curved portions 38. As shown, the radially extending portions 36 extend in the radial direction 32 but do not extend in the longitudinal direction 30, with the exception of their thickness. The curved portions 38 extend both in the longitudinal direction 30 and the radial direction 32. It is to be understood that the arrangement of the expansion element 34 shown is but one example and that others are possible in accordance with other exemplary embodiments. The expansion element 34 may be made from a variety of materials. In accordance with certain exemplary embodiments the expansion element 34 is made from stainless steel.

A connecting member 52 is present and functions to attach the expansion element 34 to the end 48 of the outer tube 16. The connecting member 52 may be made of a material different than that of the outer tube 16 or may be made of the same material. The connecting member 52 may be made out of steel, aluminum or glass in accordance with various exemplary embodiments. The connecting member 52 may be attached to the inner wall 20 so that a portion of the connecting member 52 does not extend beyond the end 48. In other arrangements, the connecting member 52 is attached to the end 48 of the outer tube 16 so that the entire connecting member 52 extends beyond the outer tube 16 in the longitudinal direction 30. In this regard, no portion of the connecting member 52 contacts or is connected to the outer wall 18 or the inner wall 20 of the outer tube 16. Connection between the connecting member 52 and end 48 may be effected though welding, soldering or mechanical fasteners. Further, these two components may be attached by being integrally formed with one another in accordance with other embodiments. In accordance with one exemplary embodiment the connection is a metal to glass joint in which the end 48 is melted glass and the connecting member 52 is oxidized metal. The connecting member 52 extends away from the end 48 in the longitudinal direction 30 such that it moves in the radial direction 32 closer to the axis 28. The connecting member 52 is attached to the expansion element 34 such that the entire expansion element 34 is located longitudinally beyond the entire connecting member 52 in the longitudinal direction 30 with respect to the outer tube 16.

The absorber tube 14 also includes a cap 54 that is attached to the outer wall 24 of the inner tube 22. The attachment may be effected through welding, soldering, mechanical fasteners, or through integral formation in accordance with various exemplary embodiments. Cap 54 has a longitudinally extending portion 56 that extends completely in the longitudinal direction 30 with the exception of the thickness of the longitudinally extending portion 56. The longitudinally extending portion 56 is attached to the expansion element 34 such that the entire longitudinally extending portion 56 is located beyond the entire expansion element 34 in the longitudinal direction 30 with respect to the outer tube 16. In accordance with certain exemplary embodiments, the entire cap 54 is located beyond the entire expansion element 34 in the longitudinal direction 30 with respect to the outer tube 16. The cap 54 and connecting member 52 may be made out of material that is different than the expansion element 34. In this regard, the cap 54 and connecting member 52 may thermally expand at a different rate than the expansion element 34. However, the three components 54, 34 and 52 may be made out of the same material and may thermally expand at the same rate in accordance with certain exemplary embodiments. Both of the ends of the absorber tube 14 may be constructed in an identical manner. However, it is to be understood that the two ends of absorber tube 14 can be configured differently in accordance with various exemplary embodiments.

FIG. 3 illustrates thermal expansion of the absorber tube 14 shown in FIG. 2. Here, solar energy transferred through the outer tube 16 is directed into the inner tube 22 that increases in temperature and thus expands thermally in the longitudinal direction 30. The outer tube 16 is not heated to the extent of inner tube 22 and thus does not expand thermally to the same degree as inner tube 22. Expansion of the inner tube 22 causes the attached expansion element 34 to expand as shown in FIG. 3. The expansion element 34 thus allows the inner tube 22 and outer tube 16 to slide relative to one another to compensate for differences in thermal expansion rates. The expansion element 34 is made of a material and provided in a certain thickness and orientation that affords some degree of flexing so that the expansion element 34 can extend and contract in the longitudinal direction 30. Once the temperature difference between the outer tube 16 and the inner tube 22 is minimized, the two tubes 16 and 22 will contract to their non-heated position. The expansion element 34 is capable of contracting so as to allow the tubes 16 and 22 to slide relative to one another. The expansion element 34 may be arranged so that is biased into a contracted position. In this regard, the expansion element 34 may expand but it will be inherently mechanically biased so as to resist expansion and act to pull the tubes 16 and 22 back into their initial position. However, the expansion element 34 need not be biased towards one position or another in accordance with various exemplary embodiments.

The space 50 present between the outer wall 24 of the inner tube 22 and the inner wall 20 of the outer tube 16 is evacuated so that a vacuum is formed therein. Space 50 may also include the area between the outer wall 24 and the longitudinally extending portion 56, connecting member 52 and expansion element 34. Provision of a vacuum increases insulation properties of the absorber tube 14. In this regard, thermal energy transferred into the inner tube 22 causes the inner tube 22 to increase in temperature. The vacuum within space 50 functions to prevent this heat from being transferred through space 50 and into the outer tube 16 and out of the absorber tube 14. As such, the inner tube 22 may be extremely hot while the outer tube 16 is cool to the touch. The vacuum thus increases the efficiency of the absorber tube 14. During manufacture of the absorber tube 14, a hole can be present in the outer tube 16 through which gases in the space 50 can be removed to achieve vacuum. The hole can be closed with a melted glass seal 58 once the gases have been removed.

A getter 40 may be provided in order to absorb or reduce gas within the space 50 so that the vacuum is maintained as strong as possible to increase efficiency of the absorber tube 14. A plate 42 can extend from a longitudinally extending portion 56 of the expansion element 34. A getter trough 44 can be defined on the plate 42 for use in carrying the getter 40. Getter 40 may be made of barium in accordance with certain exemplary embodiments. However, it is to be understood that getter 40 can be made of various materials in accordance with other embodiments. For example, getter 40 may be made of aluminum, magnesium, calcium, zirconium, phosphorus, and/or sodium in accordance with various exemplary embodiments. After evacuation of gases from the space 50, the melted glass seal 58 may be formed and the getter 40 can be heated or otherwise treated so as to effect gas absorption. For example, the getter 40 can be heated by way of radio frequency induction heating. In other embodiments, getter 40 can be heated by way of introduction into a high frequency magnesia field. The absorber tube 14 may be treated so that the getter 40 is heated or treated while the majority of the other portions of the absorber tube 14 are not treated.

Treatment of the getter 40 may cause the getter 40 to completely evaporate from the getter trough 44. The treatment process may cause the getter 40 to absorb or react with any gases remaining in the space 50 so that a more perfect vacuum is achieved. Treatment of the getter 40 may cause a coating 46 to be formed on a portion of the inner wall 20 of the outer tube 16 as shown in FIG. 4. The coating 46 may be a silver-colored metallic deposit in accordance with certain exemplary embodiments. The getter 40 and/or coating 46 will have a working life to absorb gases present within the space 50 so that a more perfect vacuum is achieved. The getter 40 may be present in getter tough 44 in addition to the presence of coating 46. However, in other embodiments, the getter 40 will be completely evaporated so that only coating 46 is present. In yet other embodiments, getter 40 will be present in getter trough 44 and coating 46 will not be present. Gases absorbed by the getter 40 may include air, carbon dioxide, carbon monoxide, nitrogen, oxygen, water and/or hydrogen.

Coating 46 may be reactive with oxygen. As such, should the absorber tube 14 leak such that air is introduced into space 50 oxygen present within the air will cause coating 46 to oxidize. Oxidization of coating 46 may cause coating 46 to turn white thus providing an indication to personnel that the absorber tube 14 has lost vacuum and is in need of repair or replacement in order to reestablish desired operating efficiency. Barium getter 40 may result in a coating 46 that becomes white when exposed to air. Barium getter 40 may continue to function to absorb gases once exposed to air. Other types of getters 40 can be used as discussed and may cause coatings 46 that turn colors other than white when the vacuum is compromised.

Getter trough 44 may be arranged so that the getter 40 is located completely within the space 50 defined between the outer wall 24 of the inner tube 22 and the inner wall 18 of the outer tube 16. In this regard, the getter 40 may be positioned so that it does not extend beyond the end 48 of the outer tube 16 in the longitudinal direction 30. However, other embodiments are possible in which the getter 40 extends partially or is completely located beyond the end 48 of the outer tube 16 in the longitudinal direction 30. Although described as being connected to expansion element 34, getter trough 44 can extend from other portions of the absorber tube 14 for example the outer tube 16, cap 54, inner tube 22, or connecting member 52 in other embodiments. Further, getter 40 need not be carried by a getter trough 44 in other arrangements and this component is thus not present in other versions of the absorber tube 14.

Getter 40 may be a solar barrier and could thus hinder the absorption of solar energy into the inner tube 22. In this regard, the getter 40 may decrease the efficiency of the parabolic trough 10. Getter 40 can thus be arranged within the absorber tube 14 so that its impact on preventing solar absorption is minimized or reduced. In this regard, getter 40 can be provided in two pieces that are oriented at 180° with respect to one another about axis 28. The two part getter 40 may thus be arranged to minimize interference with the absorption of solar energy by the inner tube 22. In other embodiments, pieces of getter 40 can be located 90°-180° from one another about axis 28. In yet other arrangements, pieces of getter 40 can be located up to 45° from one another. In other versions of the absorber tube 14, getter 40 may be a single piece and can be located 360° about the axis 28. Further, getter 40 can be placed proximate to the expansion element 34 so that it covers a minimum amount of the inner tube 22 that is exposed to solar energy though the outer tube 16. Although shown in FIG. 2 as being located at both ends of the absorber tube 14, it is to be understood that the getter 40 need not be located at both ends but may be located only at a single end of the absorber tube 14 in certain exemplary embodiments. Such an arrangement may act to minimize the surface area of getter 40 and thus minimize the resulting decrease in efficiency of the absorber tube 14. The coating 46 may be limited to an end of the outer tube 16 so that interference of thermal energy transmission into the inner tube 22 is minimized. Coating 46 may be arranged so that it is generally two sections that are located 180° from one another about axis 28. However, it is to be understood that coating 46 may extend across any portion of the outer tube 16 in other embodiments.

While the present invention has been described in connection with certain preferred embodiments, it is to be understood that the subject matter encompassed by way of the present invention is not to be limited to those specific embodiments. On the contrary, it is intended for the subject matter of the invention to include all alternatives, modifications and equivalents as can be included within the spirit and scope of the following claims. 

1. An absorber tube, comprising: an inner tube; an outer tube surrounding at least a portion of the inner tube and concentric with the inner tube; an expansion element configured for expanding in response to differences in thermal expansion between the inner tube and the outer tube; and a getter located radially at a position between the inner tube and the outer tube.
 2. The absorber tube as set forth in claim 1, wherein the getter is located at a position between the inner tube and the outer tube such that the getter does not extend longitudinally beyond an end of the outer tube.
 3. The absorber tube as set forth in claim 1, wherein the getter is located at a position between the inner tube and the outer tube such that at least a portion of the getter extends longitudinally beyond an end of the outer tube.
 4. The absorber tube as set forth in claim 1, wherein the getter is a barium getter.
 5. The absorber tube as set forth in claim 1, further comprising a plate that is attached to the expansion element and that defines a getter trough, wherein the getter is carried by the getter trough.
 6. The absorber tube as set forth in claim 1, wherein the getter includes two pieces that are located at an angle of 180° from one another about the concentric axis of the inner tube and the outer tube.
 7. The absorber tube as set forth in claim 1, wherein at least a portion of the getter is located on the inner wall of the outer tube and is configured to change colors upon the introduction of air into the location between the inner tube and the outer tube.
 8. The absorber tube as set forth in claim 1, further comprising: a connecting member attached to the end of the outer tube and extending from the end of the outer tube such that the connecting member is located completely longitudinally beyond the outer tube, wherein the connecting member is attached to the expansion element; and a cap attached to the inner tube, wherein the cap has a longitudinally extending portion that is attached to the expansion element; wherein the expansion element has a series of radially extending portions connected by curved portions, wherein the curved portions extend in both the radial and longitudinal directions, and wherein the entire expansion element is located beyond the end of the outer tube in the longitudinal direction.
 9. An absorber tube, comprising: an inner tube; an outer tube at least partially surrounding the inner tube and concentric with the inner tube; a connecting member attached to an end of the outer tube; and an expansion element attached to the connecting member, wherein the expansion element is configured for expanding in response to differences in thermal expansion between the inner tube and the outer tube, wherein the expansion element is located completely beyond the end of the outer tube in the longitudinal direction.
 10. The absorber tube as set forth in claim 9, wherein a vacuum is present between the inner tube and the outer tube, and wherein the outer tube is made at least partially of glass.
 11. The absorber tube as set forth in claim 9, wherein thermal expansion of the inner tube at a rate faster than thermal expansion of the outer tube causes the expansion element to expand in the longitudinal direction; and wherein thermal contraction of the inner tube at a rate faster than thermal contraction of the outer tube causes the expansion element to contract in the longitudinal direction.
 12. The absorber tube as set forth in claim 9, further comprising a cap attached to the inner tube, wherein the cap has a longitudinally extending portion that is attached to the expansion element, wherein the expansion element is located completely beyond the connecting member in the longitudinal direction with respect to the outer tube, and wherein the longitudinally extending portion of the cap is located completely beyond the expansion element in the longitudinal direction with respect to the outer tube.
 13. The absorber tube as set forth in claim 9, wherein the expansion element has a series of radially extending portions connected by curved portions, wherein the curved portions extend in both the radial and longitudinal directions.
 14. The absorber tube as set forth in claim 9, further comprising a getter located radially at a position between the inner tube and the outer tube, wherein the getter is located at a position between the inner tube and the outer tube such that the getter does not extend longitudinally beyond the end of the outer tube.
 15. The absorber tube as set forth in claim 14, wherein the getter is a barium getter and further comprising a plate that is attached to the expansion element and that defines a getter trough, wherein the getter is carried by the getter trough.
 16. An absorber tube, comprising: an inner tube; an outer tube at least partially surrounding the inner tube and concentric with the inner tube; a cap attached to the inner tube, wherein the cap has a longitudinally extending portion that extends in the longitudinal direction; a connecting member attached to an end of the outer tube; and an expansion element attached to the longitudinally extending portion of the cap, wherein the expansion element is attached to the connecting member, wherein the expansion element is located completely between the longitudinally extending portion of the cap and the connecting member in the longitudinal direction.
 17. The absorber tube as set forth in claim 16, wherein thermal expansion of the inner tube at a rate faster than thermal expansion of the outer tube causes the expansion element to expand in the longitudinal direction; wherein thermal contraction of the inner tube at a rate faster than thermal contraction of the outer tube causes the expansion element to contract in the longitudinal direction; wherein the expansion element is located completely beyond the end of the outer tube in the longitudinal direction; wherein the connecting member extends from the end of the outer tube such that the connecting member is located completely longitudinally beyond the outer tube.
 18. The absorber tube as set forth in claim 16, wherein a vacuum is formed in the space between the outer wall of the inner tube and the inner wall of the outer tube, and further comprising a getter located within the space between the outer wall of the inner tube and the inner wall of the outer tube.
 19. The absorber tube as set forth in claim 18, wherein the getter includes two pieces that are located at an angle of 180° from one another about the concentric axis of the inner tube and the outer tube.
 20. The absorber tube as set forth in claim 18, wherein at least a portion of the getter is located on the inner wall of the outer tube and is configured to change colors upon the introduction of air into the space between the outer wall of the inner tube and the inner wall of the outer tube. 